1. Technical Field
The present invention relates to a fuel injection valve such as a pilot fuel injection device in a gas engine, or a fuel injection device in a diesel engine, whereby the injection valve injects fuel supplied in a fuel pool surrounding a needle valve into an engine cylinder. The injection valve stops the fuel injection in a way such that a fuel flow channel between a tip of the needle valve and a valve seat of a nozzle tip is opened or closed by a reciprocating movement of the needle valve that is fitted slidably and guided in the nozzle tip.
2. Related Art
As is shown in patent reference 1 (JP2002-295342), a fuel injection valve applied to a pilot fuel injection device in a gas engine, a fuel injection device in a diesel engine, or the like, injects fuel supplied in a fuel pool surrounding a needle valve into an engine cylinder through at least one nozzle hole provided at a tip of a nozzle tip, and also stops the injection, using a fuel flow passage between a seat surface of the needle valve tip and a seat surface of the nozzle tip which is opened/closed by reciprocating movements of a needle valve fitted slidably in a borehole formed in the nozzle tip.
In a medium size or large size gas engine for generators and diesel engines which are provided with the fuel injection valve as mentioned above, diesel oil is frequently used as a fuel (in large marine-diesel engines, heavy fuel oil and/or heavy duty fuel is usually used).
The fuel used in the fuel injection valves for diesel fuel is apt to contain relatively large amounts of foreign substances, causing the foreign substances to often enter a sliding clearance around the needle valve. Moreover, impurities are sometimes introduced into fuel oil during engine maintenance, and the fuel oil introduces the impurities into the sliding clearance around the needle valve. The above-mentioned foreign substances or impurities on the sliding surfaces often cause a malfunction, wear or seizure of the needle valve.
When burnt, diesel oil generates more combustion residues than gas oil does; thus, in an injection valve of an engine that is operated with diesel oil, a considerable amount of combustion residue accumulates in the neighborhood of the valve-seat surfaces around the needle valve, which easily induces poor combustion caused by irregular injections because of the accumulated hard residues exfoliating from the surfaces and scratching the surfaces.
Patent reference 1 (JP2002-295342) discloses a needle valve that has a plurality of radial grooves around the outer periphery of the needle valve so as to improve a lubricating condition between the periphery of the needle valve and the nozzle tip.
Patent reference 2 (JP2005-533222) discloses a fuel injection valve that has a plurality of micro-depressions configured on the seat surfaces of the needle valve tip and/or the nozzle tip so as to improve the lubricating condition mentioned above using fuel held in the depressions as a lubricant.
As mentioned above, in fuel injection valves which use a fuel such as diesel oil containing a relatively large amount of foreign substances, the foreign substances or impurities introduced by the fuel oil itself or through engine maintenance work often enter the sliding clearance around the needle valve. Moreover, the diesel oil yields a considerable amount of combustion residue which is apt to damage the seat surfaces of the needle valve tip and/or the nozzle tip.
FIG. 11 shows an example of foreign substance distribution in a diesel oil sample and a gas oil sample. The data shows that the distribution quantity of the foreign substances in the diesel oil is 10 to 100 times higher than that in the gas oil when a particle size of the foreign substances is 5 to 15 μm.
In the disclosure of patent reference 1, even though the needle valve is provided with a plurality of radial grooves around the outer periphery of the needle valve for a purpose of improving a lubricating condition between the periphery of the needle valve and the nozzle tip, the purpose of the grooves is mainly to improve lubrication by means of retaining fuel oil in the grooves. Thus, each groove is not open to the outside, so foreign substances and/or impurities that are brought therein are apt to enter the sliding clearance around the needle valve through the reciprocating movements of the needle valve. Therefore, even with the technology disclosed in the patent reference 1, intrusion of foreign substances and/or impurities into the sliding clearance around the needle valve still easily occurs, and the problems of malfunctions, wear or seizure of the needle valve periphery remain unsolved.
Further, in the disclosure of patent reference 2, a plurality of micro-depressions are configured on the seat surfaces of the needle valve tip and/or the nozzle tip so as to improve the lubricating condition by the fuel oil remaining in the depressions as a lubricant. However, the seat surface of the needle valve tip comes in contact with the seat surface of the nozzle tip so that the areas of contacting surfaces are kept substantially unchanged. Thus, when combustion residues, foreign substances, impurities, and so on, intrude into the seat areas, those solid foreign-matters are not removed from the micro-depressions, even if the depressions improve the lubrication. Consequently, there arise problems such as irregular fuel injections and resulting poor combustion, in response to the damage of the solid foreign matters on the seat surfaces.